In the electronics field, there has been a strong inclination towards “small.” Therefore, people are keen to integrate more components on a single printed circuit board. This necessitates the continual compression of space and enhancement of space utilization on the boards. Traditional through-hole vias are inadequate for these demands, leading to the emergence of an innovative technology – microvias. As miniaturization and portable devices become increasingly prevalent, the significance of this technology is expected to rise. Thus, whether you are an experienced designer or a curious enthusiast, it is essential to delve deeper into it!
Definition of Microvia
Microvias, as the name suggests, are very small holes. This is a vague concept – what size qualifies as “micro”? According to the latest guidelines revised by IPC in 2013, microvias are defined as holes with a depth-to-diameter ratio of less than 1:1 and a depth not exceeding 0.25mm.
Microvias are typically used to connect selected layers rather than penetrate and connect all layers, facilitating high-density interconnections between the internal layers of PCB. This provides greater freedom in circuit design and drives the densification of new packaging on PCB. However, due to the small size of the holes, their formation is challenging and often requires advanced laser technology.
Reliability of Microvias
Although microvias are crucial for HDI technology, their reliability has long been a concern due to their small size and the need to withstand complex thermo-mechanical environments. Research focuses on experimental evaluation of unfilled single-layer microvias and finite element analysis to explore stress and strain distributions within microvia structures, as well as fatigue life.
Studies have found that the main factors affecting the reliability of microvias stem from the thermo-mechanical stress experienced in the vertical direction of the PCB, caused by the mismatch in coefficient of thermal expansion (CTE) between the metal in the microvia structure and the surrounding dielectric material. Potential failures may include interface separation, barrel cracks, corner/knee cracks, and target pad cracks.
To ensure the reliability of microvias, attention must be paid to designing them to meet specifications in terms of diameter, selecting substrates with sufficient mechanical strength and thermal stability, and considering board thickness and hole wall proportions. During processing, it is necessary to keep the inner wall of the hole smooth and uniform, avoiding burrs, cracks, or other defects.
Staggered vs. Stacked vs. Skipped
| Feature | Staggered Microvias | Stacked Microvias | Skipped Microvias |
|---|---|---|---|
| Definition | Microvias connecting different layers | Multiple holes stacked vertically | Connections skipping certain layers |
| Structure | Single hole connecting different layers | Multiple holes stacked vertically | Skipping connections to certain layers |
| Advantages | Suitable for general designs and lower cost | Provides higher connection density | Used for simplifying routing or avoiding interference |
| Disadvantages | Connections limited to hole positions | Higher manufacturing costs | Precise control of signal paths required |
Microvia Processing Techniques
Microvia processing lies between traditional and microfabrication due to its small diameter and high difficulty. Currently, the most widely used techniques include mechanical and laser (including CO2 and UV) methods.
In mechanical processing:
- Maximum speed can reach 350,000 rpm;
- Machining diameters exceed 0.1mm;
- Hole wall roughness is generally less than 30μm;
- Hole diameter tolerance is generally less than +10μm/-40μm;
- High demands on hole position accuracy are generally less than ±50μm.
In laser processing:
- Machining diameters range from 0.075mm to 0.1mm;
- Hole wall roughness is generally less than 18μm;
- Hole diameter tolerance is generally less than ±20μm;
- High demands on hole position accuracy are generally less than ±20μm.
Although mechanical processing may not match laser processing in terms of dimensional accuracy, it still holds unique advantages in quality and through-hole processing capabilities. With the application of high-speed drilling machines, the efficiency of mechanical drilling has also been significantly improved.
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